From what I read about Ptolemy, he wasn't murderer or usurper. He agreed with reign of Philipos Arrhidaios and than Alexander IV, he believed that Alexanders empire cannot be maintained and wasn't interested in wars to control this empire, he was interested only in Egypt although he didn't miss a chance to conquer former Egyptian territories or move borders of his kingdom. After unsuccessful campaign of Perdikas to Egypt he brought wheat and meat to his soldiers and refused to be titled a king claiming that he was only a satrap of Egypt. There are more examples of his generosity and unselfishness and he doesn't fit into the portrait of murderer. If only this wasn't the reason, the relief how AP claimed.